The expression “on-line generation” of data during a reaction is used herein to denote generation of the data sufficiently rapidly so that the data is available essentially instantaneously or sometime thereafter for use during the reaction. The expression “generation of data in on-line fashion” during a reaction is used synonymously with the expression on-line generation of data during a reaction. Generation of data from a laboratory test (on at least one substance employed or generated in the reaction) is not considered “on-line generation” of data during the reaction, if the laboratory test consumes so much time that parameters of the reaction may change significantly during the time required to conduct the test. It is contemplated that on-line generation of data may include the use of a previously generated database that may have been generated in any of a variety of ways including time-consuming laboratory tests.
With reference to a product being produced by a continuous reaction, the expression “instantaneous” value of a property of the product herein denotes the value of the property of the most recently produced quantity of the product. Because of the back-mixed nature of a gas phase polymerization reactor, the most recently produced polymer product typically undergoes mixing with previously produced quantities of product before a mixture of the recently and previously produced product exits the reactor. In contrast, with reference to a product being produced by a continuous reaction, the expression “average” (or “bed average”) value (at a time “T”) of a property herein denotes the value of the property of the product that exits the reactor at time T.
Throughout this disclosure, the expression “diluent” (or “condensable diluent” or “condensable diluent gas”) denotes condensable gas (or a mixture of condensable gases) present in a polymerization reactor with polymer resin being produced. The diluent is condensable at the temperatures encountered in the process heat exchanger. Examples of diluents include induced condensing agents (“ICAs”), comonomers, isomers of comonomers, and combinations thereof.
The expression “dry polymer resin” (or “dry version” of polymer resin) is used herein to denote polymer resin that does not contain substantial amounts of dissolved gas. An example of dry polymer resin is polymer that had been previously produced in a polymerization reactor and then purged to eliminate all (or substantially all) unreacted monomers, comonomers and ICAs that had been dissolved in the polymer at the time of production. As will be discussed herein, a dry version of polymer resin has significantly different melting (and sticking) behavior than would the same polymer resin if it were in the presence of a significant amount of condensable diluent gas and comonomer.
The expression “polyethylene” denotes at least one polymer of ethylene and optionally one or more C3-C10 α-olefins, while the expression polyolefin denotes at least one polymer (or copolymer) of one or more C2-C10 α-olefins.
Throughout this disclosure, the abbreviation “MI” denotes melt index (I2) of the polymer product, measured in accordance with ASTM-D-1238-E unless otherwise stated. Also throughout this disclosure, the term “density” denotes the intrinsic material density of a polymer product (in units of g/cc), measured in accordance with ASTM-D-1505-98 unless otherwise stated.
One method for producing polymers is gas phase polymerization. A conventional gas phase fluidized bed reactor, commonly employs a fluidized dense-phase bed typically including a mixture of reaction gas, polymer (resin) particles, catalyst, and (optionally) other additives. Typically, any of several process control variables will cause the reaction product to have certain, preferably desired, characteristics.
Generally in a gas-phase fluidized bed process for producing polymers from monomers, a gaseous stream containing one or more monomers is continuously passed through a fluidized bed under reactive conditions in the presence of an activated catalyst. This gaseous stream is optionally withdrawn from the top of the fluidized bed and recycled back into the reactor. Simultaneously, polymer product is withdrawn from the reactor and new monomer is added to replace the polymerized monomer. The recycled gas stream is heated in the reactor by the heat of polymerization. This heat is typically removed in another part of the cycle by a cooling system external to the reactor.
It is important to remove heat generated by the reaction in order to maintain the temperature of the resin and gaseous stream inside the reactor at a temperature below the polymer melting point and/or catalyst deactivation temperature. Further, heat removal is important to control the reactor temperature to prevent excessive stickiness of polymer particles that if left unchecked, may result in loss of fluidization or agglomeration of the sticky particles which may lead to formation of chunks or sheets of fused polymer that cannot be removed as product. The production of such chunks or sheets can present significant operational problems in fluidized bed reactor systems because, once formed, the fused chunks or sheets may fall onto the distributor plate causing impaired fluidization and mixing, which in many cases requires a reactor shutdown for cleaning. Prevention of such excessive resin stickiness has been accomplished by controlling the temperature of the fluid bed to a temperature just below the fusion or sintering temperature of the polymer particles. Above this fusion or sintering temperature, empirical evidence suggests that such fusion or sintering leads to agglomeration or stickiness in the polymer product, which can in turn (if left unchecked) lead to the above conditions.
In addition, the amount of polymer produced in a fluidized bed polymerization process is directly related to the amount of heat that can be withdrawn from the fluidized bed reaction zone. In steady state operation of the reaction process, ideally, the rate of heat removal from the fluidized bed must equal the rate of rate of heat generation, such that the bed temperature remains constant. Conventionally, heat has been removed from the fluidized bed by cooling the gas recycle stream in a heat exchanger external to the reactor.
A requirement of a fluidized bed process is that the velocity of the gaseous recycle stream be sufficiently high to maintain the reaction zone in a fluidized state. In a conventional fluidized bed polymerization process, the amount of fluid circulated to remove the heat of polymerization is greater than the amount of fluid required for support of the fluidized bed and for adequate mixing of the solids in the fluidized bed. The excess velocity provides additional gas flow to (and through) the fluid bed for additional cooling capacity and more intensive mixing of the reactor bed. However, to prevent excessive entrainment of solids in a gaseous stream withdrawn from the fluidized bed, the velocity of the gaseous stream must be regulated.
For a time, it was thought that the temperature of the gaseous stream external to the reactor, otherwise known as the recycle stream temperature, could not be decreased below the dew point of the recycle stream without causing problems of polymer agglomeration or plugging of the reactor system. The dew point of the recycle stream is that temperature at which liquid condensate first begins to form in the gaseous recycle stream. The dew point may be calculated knowing the gas composition, and may be thermodynamically defined.
Contrary to this belief, as suggested by Jenkins et al. in U.S. Pat. Nos. 4,543,399 and 4,588,790, a recycle stream can be cooled to a temperature below the dew point in a fluidized bed polymerization process resulting in condensing a portion of the recycle gas stream. The resulting stream containing entrained liquid can be returned to the reactor without causing the aforementioned agglomeration and/or plugging phenomena (which was generally expected prior to Jenkins). The process of purposefully condensing a portion of the recycle stream is known in the industry as “condensed mode” operation in a gas phase polymerization process.
The above-cited U.S. patents to Jenkins et al. suggest that when a recycle stream temperature is lowered to a point below its dew point in “condensed mode” operation, an increase in polymer production is possible, as compared to production in a non-condensing mode because of increased cooling capacity. Consequently, a substantial increase in space-time yield, the polymer production rate per unit of reactor volume, can be achieved by condensed mode operation with little or no change in product properties.
Cooling of the recycle stream to a temperature below the gas dew point temperature produces a two-phase gas/liquid mixture with solids contained in both of these phases. The liquid phase of this two-phase gas/liquid mixture in condensed mode operation remains entrained or suspended in the gas phase portion of the mixture. Vaporization of the liquid occurs only when heat is added or pressure is reduced. In this process, vaporization occurs when the two-phase mixture enters the fluidized bed, with the (warmer) resin providing the required heat of vaporization. The vaporization thus provides an additional means of extracting heat of reaction from the fluidized bed. The heat removal capacity is further enhanced in condensed mode operation by the increased (sensible) heat transfer associated with the lower temperatures of the gas stream entering the fluidized bed. Both of these factors increase the overall heat removal capability of the system and thereby enable higher space-time yields.
Jenkins, et al. illustrate the difficulty and complexity of such condensed mode reaction control in general, and of trying to extend the stable operating zone to optimize the space time yield in a gas phase reactor, especially when operating in condensed mode.
The cooling capacity of recycle gas can be increased further while at a given reaction temperature and a given temperature of the cooling heat transfer medium. One option described is to add non-polymerizing, non-reactive materials to the reactor, which are in the gaseous state in the fluidized bed section of the reactor, but are condensable at the lower temperatures encountered in the process heat exchanger. Such non-reactive, condensable materials are collectively known as induced condensing agents (ICAs) since they “induce” additional condensing in the system. Increasing concentrations of ICA in the reactor causes corresponding increases in the dew point temperature of the reactor gas, which (for a given heat exchanger temperature) promotes higher levels of condensing for higher heat-transfer limited production rates from the reactor. Suitable ICA materials are selected based on their specific heat and boiling point properties. In particular, ICA compounds are selected such that a relatively high portion of the material is condensed at the cooling water temperatures available in polymer production plants, which are typically 20-40° C. ICA materials include hexane, isohexane, pentane, isopentane, butane, isobutane and other hydrocarbon compounds that are similarly non-reactive in the polymerization process.
U.S. Pat. No. 5,352,749, teaches, among other things, that there are limits to the concentrations of condensable gases, whether ICA materials, comonomers or combinations thereof, that can be tolerated in the reaction system. Above certain limiting concentrations, the condensable gases can cause a sudden loss of fluidization in the reactor, and a consequent loss in ability to control the temperature in the fluid bed. U.S. Pat. Nos. 5,352,749, 5,405,922 and 5,436,304, suggest upper limits of ICA in the reactor, depending on the type of polymer being produced. The authors characterized the upper limit of condensable materials by tracking the ratio of fluidized bulk density to settled bulk density. As the concentration of isopentane (ICA) was increased in an otherwise steady-state reaction, they found that the bulk density ratio steadily decreased. When the concentration of isopentane was sufficiently high, corresponding to a bulk density ratio of 0.59, they found that fluidization in the reactor was lost. They, therefore, determined that this ratio (0.59) represented a limiting value below which a reactor would cease functioning due to loss of fluidization.
As described in U.S. Pat. No. 7,122,607, attempts to operate polymerization reactors with excessive ICA concentrations cause polymer particles suspended in the fluid bed to become cohesive or “sticky,” and in some cases cause the fluid bed to solidify in the form of a large chunk. This stickiness problem is characterized by undesirable changes in fluidization and mixing in the fluid bed, which if left unchecked, may develop into a reactor discontinuity event, such as sheeting or chunking. Chunks are solid masses of polymer that can form within the interior of the fluidized bed. Sheets are solid masses of polymer that can form on the interior reactor walls. The sheets eventually become dislodged from the walls and fall into the reaction section. These solid masses of polymer (sheets or chunks) may settle on the distributor plate, where they interfere with fluidization, block the product discharge port, and usually force a reactor shut-down for cleaning. The lost production associated with such forced reactor shut-downs can have significant economic impact in large-scale, commercial production plants.
There are at least two distinct types of sheets that can be formed in gas phase reactors: wall sheets or dome sheets, depending on where they are formed in the reactor. Wall sheets are formed on the walls (generally vertical walls) of the reaction section. Dome sheets are formed higher in the reactor, on the conical section of the dome, or on the hemispherical head on the top of the reactor.
When sheeting occurs with Ziegler-Natta catalysts, it is generally wall sheeting in the lower portion of the reaction section. Ziegler-Natta catalysts are capable of forming dome sheets, but the occurrence is rare. With metallocene catalysts, however, sheeting can occur in either or both locations (i.e. both wall sheeting and dome sheeting can occur). Dome sheeting has been particularly troublesome with metallocene catalyst systems.
The term “discontinuity event” is used to describe a forced disruption in the continuous operation of a polymerization reactor caused by, e.g., wall or dome sheeting, chunking or fouling of the gas recycle system. The terms “sheeting and/or chunking” while used synonymously herein, may describe different manifestations of problems discussed herein. In either manifestation (sheeting or chucking), the excessive polymer stickiness may lead directly to a reactor discontinuity event with the associated loss production.
Throughout this disclosure, the terms “fusion temperature,” “sintering temperature,” and “sticking temperature” are used synonymously to denote the temperature at which the polymer in the reactor (in the presence of reaction and diluent gases) reaches conditions of limiting stickiness, which can lead to the discontinuity events described above.
Two articles by Process Analysis & Automation Limited (PAA), entitled “Agglomeration Detection by Acoustic Emission,” PAA Application note: 2002/111 (2000) and “Acoustic Emission Technology—a New Sensing Technique for Optimising Polyolefin Production” (2000), suggest process control in fluidized bed production of polyolefins utilizing acoustic emission sensors located at various positions on the reactor and recycle piping. These publications purport to solve the problem of detecting the presence of large polymer agglomerates in a reactor, such as chunks or sheets, rather than detecting stickiness of the resin particles. One specific example is provided showing the detection of a chunk of approximately 1.5 meters in diameter within a commercial fluid bed reactor.
WO 03/051929 describes the use of mathematical chaos theory to detect the onset and presence of sheeting in a fluid bed reactor. Signals from a range of instruments, including acoustic emission sensors, differential pressure sensors, static sensors, and wall temperature sensors are filtered by certain specified methods to construct a “time-series” of data, which is then processed by methods of non-linear dynamics (herein referred to as chaos theory) and compared to data from a control reactor running without sheeting. The onset of sheeting is indicated by an increase in mean “cycle time” (relative to a baseline, control reactor), usually with a concurrent decrease in the “mean deviation” of the time-series. Alternatively, the onset of sheeting is indicated by a decrease in the mathematical “entropy” of the time-series data, as compared to a similar reactor running without sheeting. (The terms “time-series,” “cycle time,” “mean deviation,” and “entropy” here refer to calculated parameters defined by chaos theory).
Adding to the complexity of control of stickiness while using ICAs, different polymer products vary widely in their ability to tolerate ICA materials, some having a relatively high tolerance (expressed in partial pressure of the ICA in the reactor). For example, some polymers can tolerate as much as 50 psia of ICA, while other polymers can tolerate only 5 psia or less. With these latter polymers, the heat-transfer limited production rates under similar conditions are substantially lower. Polymers which possess a more uniform comonomer composition distribution are known to have a higher tolerance to the partial pressure of the ICA in the reactor. Metallocene catalyst produced polymers are a good example of polymers with such a more uniform comonomer composition. However, at some point even these metallocene produced polymers reach a limiting ICA concentration that induces stickiness. The limiting ICA concentration depends on several factors in addition to the polymer type, including reactor temperature, comonomer type and concentration, etc. Further, with the effect of temperature, ICA level and comonomer levels all affecting on the onset of stickiness, determining the point at which sticking begins to occur has heretofore been difficult.
Even within the constraints of conventional, safe operation, control of such reactors is complex adding further to the difficulty and uncertainty of experimentation if one wishes to find new and improved operating conditions that might result in higher production rates. Discontinuity events at large-scale, gas phase polymer production plants are expensive. Further, risks associated with experimentation in such plants are high due to the high cost of reactor downtime. Therefore, it is difficult to explore design and operating boundaries experimentally in view of the costs and risks involved.
It would be desirable to provide a method of determining a stable operating condition for gas fluidized bed polymerization, especially if operating in condensed mode, to facilitate optimum design of the plant and the determination of desirable process conditions for optimum or maximum production rates in a given plant design.
It would also be desirable to have a mechanism in commercial gas-phase reactors to detect the onset of stickiness (in on-line fashion) that is a better or an earlier indicator of the onset of stickiness than are conventional techniques (e.g., monitoring the fluidized bulk density as described in U.S. Pat. No. 5,352,749). Such a mechanism would allow the operators to determine when conditions of limiting stickiness were being approached, and enable them to take corrective action before discontinuity events (such as sheeting and chunking) occurred, while keeping the reactors at or near conditions of maximum ICA concentration, permitting higher production rates with substantially less risk.
WO 2005/113615 and corresponding U.S. Pat. No. 7,122,607 describe the determination of a critical temperature below which resin in a polymerization reactor cannot become sticky, and use of this predetermined critical temperature to control the reactor. These references define “dry sticking temperature” of a polymer to be produced in a fluidized bed reactor as the temperature at which agglomeration within the bed or fouling on any surface of the reactor vessel begins to occur with the reactor operating at normal pressure and gas velocity but in the presence of substantially pure nitrogen, rather than the normal gas components. They define a liquid “melting point depression” as the temperature by which the melting point of the polymer in the reactor is depressed by liquid immersion of the polymer in the hydrocarbons (ICA and comonomer) to be used in the process. Because the measurements are carried out in the presence of the liquid hydrocarbons (rather than gases), the resulting “melting point depression” represents the maximum amount by which the melting point can be depressed in a reactor operating in the gas phase with the same hydrocarbon materials. The references also describe a method including the steps of determining the dry sticking temperature of a polymer to be produced; determining the melting point depression for the reaction; and then operating the gas phase reactor process with a bed temperature below a “critical temperature” defined as the dry sticking temperature minus the liquid melting point depression. The references teach that performing the reaction with the bed temperature below the critical temperature can eliminate stickiness induced in the resin due to high concentrations of condensables.
U.S. patent application Ser. No. 11/227,710, discloses on-line monitoring of resin stickiness in a polymerization reactor by generating a time series of acoustic emissions readings generated by the contents of the reactor. Acoustic emission measurements are first generated during steady state operation of a reactor (producing the relevant polymer). Additional acoustic emission measurements (generated during non-steady state operation of the reactor) are then processed to determine whether they deviate from acoustic emissions indicative of steady state reactor operation. Such deviation is treated as an indication of onset of excessive stickiness of polymer particles in the reactor. Corrective action can be taken (e.g., reductions in ICA and/or monomer concentrations and/or a reduction in reactor temperature) when the acoustic emission measurements are determined to deviate (negatively) beyond three standard deviations of those of a steady state reactor.
U.S. Patent Application Nos. 60/842,747 (“MRT application”) and 60/842,719 (“MIT application”), both filed on Sep. 7, 2006, describe methods for detecting conditions indicative of imminent occurrence of sheeting during polymerization reactions in fluid bed polymerization reactors, and preferably also controlling the reactions to prevent the occurrence of sheeting.
The MRT application describes a method including of the steps of: monitoring a polymerization reaction which produces a polymer resin in a fluid bed reactor, wherein a dry melt reference temperature is determined that is characteristic of melting behavior of a dry version of the polymer resin; and in response to data indicative of at least one monitored parameter of the reaction, determining, in on-line fashion, a reduced melt reference temperature characteristic of the melting behavior of the polymer resin as it exists in the reactor. The reduced melt reference temperature (MRTR) is at least substantially equal to the difference between the dry melt reference temperature and a melt reference temperature depression value, “D,” where D is a temperature by which the dry melt reference temperature is depressed by the presence of diluent that is present with the resin in the reactor. The method optionally also includes the steps of determining a stickiness control parameter from the reduced melt reference temperature, and controlling the reaction in response to the stickiness control parameter.
The MIT application describes a specific method of applying the MRT method, and includes of the steps of:
(a) during a polymerization reaction in a fluid bed reactor which produces a polymer resin, measuring parameters of the reaction including at least reactor temperature, at least one resin property of the polymer resin, and concentration of at least one condensable diluent gas in the reactor;
(b) determining from the at least one resin property, using a predetermined correlation, a dry melt initiation temperature of a dry version of the polymer resin; and
(c) during the reaction, using a melt initiation temperature depression model to determine, in on-line fashion from at least some of the parameters measured in step (a) and the dry melt initiation temperature value, a reduced melt initiation temperature for the polymer resin in the presence of the at least one condensable diluent gas, said melt initiation temperature depression model identifying an estimated degree of depression of the dry melt initiation temperature due to presence of at least one diluent with the polymer resin. In typical embodiments, the melt initiation temperature depression model implements the well-known Flory melt depression equation; and optionally also the step of:
(d) determining in on-line fashion a temperature value indicative of resin stickiness in the reactor, from the reduced melt initiation temperature determined in step (c) and a current value of the reactor temperature. Typically, the temperature value generated in step (d) is a temperature value ΔMIT that is at least substantially equal to Trx−MITR, where Trx is the current value of reactor temperature, and MITR is the reduced melt initiation temperature determined in step (c). The temperature value indicative of resin stickiness determined in this manner (ΔMIT) may be used in the present inventive method as a parameter to control resin stickiness in the fluidized bed.
The MIT and MRT applications also disclose methods and systems for on-line generation of data indicative of imminent occurrence of limiting resin stickiness in the fluidized bed, which if left unchecked, could lead to sheeting or other discontinuity events in the reactor. The present invention pertains to, inter alia, a reaction monitoring system that incorporates these methods for monitoring resin stickiness together with methods for monitoring other reaction parameters such as bed static and carryover static. This combination of data provides for an improved, on-line prediction of the imminent occurrence of sheeting in a fluidized bed polymerization reaction system.
The expressions “bed static” and “reactor static” are used synonymously in the present disclosure to denote the static charge that is generated by frictional contact between the resin and the reactor walls in the fluidized bed section of the reactor. Static probes suitable for measuring the bed static are described in, for example, U.S. Pat. No. 6,008,662, and further described in the present disclosure.
Above-cited related U.S. Patent Application Publication No. 2005/0148742 describes use of static probes positioned in the “entrainment zone” of a fluidized bed polymerization reaction system to monitor “carryover static.” This reference also describes control of the reaction in response to the results of such carryover static monitoring to prevent discontinuity events such as chunking and sheeting (e.g., to reduce carryover static and thereby prevent such discontinuity events).
The expression “entrainment zone” of a fluidized bed reactor system is used in US Patent Application Publication No. 2005/0148742 and the present disclosure to denote any location in the reactor system outside the dense phase zone of the system (i.e., outside the fluidized bed).
The expression “carryover static” is used in U.S. Patent Application Publication No. 2005/0148742 and the present disclosure to denote the static charging that results from frictional contact by particles (e.g., catalyst particles and resin particles) against the metal walls of the gas recycle line, or against other metal components in the reactor entrainment zone. Carryover static can be measured by suitable static probes positioned in various sections of the entrainment zone of the reaction system, including the expanded (disengagement) section, the recycle line, and the distributor plate.
In the present disclosure, the expression “entrainment static” denotes carryover static that results from frictional contact between entrained particles and a static probe located in a gas recycle line of a fluidized bed reactor system. Thus, the term “entrainment static” represents a specific means of measuring the carryover static generated by frictional contact of entrained particles that occur throughout the gas recycle system.
U.S. Pat. No. 4,532,311 describes, inter alia, the use of “skin” temperature sensors, which are configured and positioned to sense the temperature Tw of the resin and/or reactor gas near the wall of the reactor during operation. The skin temperature sensors are typically implemented as thermocouple sensors mounted in positions along straight section of the reactor wall so as to protrude a short distance into the reactor (e.g., 3 to 12 mm). These sensors are capable of detecting the formation of a fused polymer sheet at a specific location on the wall, as indicated by an increased “skin” temperature reading of typically 3 to 20° C. above their normal, steady state reading. However, the utility of these sensors is limited because they can detect only those sheets that form in the specific location of a sensor, and because they are coincident indicators. They provide an indication that a sheet is currently forming (at that location) but cannot provide early warning of an impending sheeting event. Skin temperature sensors may also be implemented near the walls of the reactor expanded section, preferably 0.1 to 1.0 reactor diameters above the operating level of the fluidized bed (as illustrated in FIG. 6). In this location, the skin temperature sensors are capable of detecting (at least in some cases) the occurrence of dome sheeting in the lower portion of the expanded section.
Other background references include WO 86/07065, WO 99/02573, WO 02/30993, WO 2006/107437, and U.S. Patent Application Publication No. 2006/130870.
It would therefore be desirable to define a system capable of providing an early indication of impending sheeting or other discontinuity event in a gas phase, fluidized bed reactor that would provide sufficient advanced warning to enable the operators to make changes in the process to avoid the associated discontinuity event. Preferably, such an “early warning” or “continuity monitoring” system would be based on real-time measurements (i.e. monitoring) of the fundamental process parameters that cause wall and dome sheeting with metallocene catalysts.